Updated piston_window to v0.120.0 so that it works with latest stable rust

[rrohrer]

As stated in an older pull request, this allows the plotters-piston repo to now run with the latest stable rust.

[alepez]

piston_window 0.112.0 is crashing with

thread 'main' panicked at 'attempted to leave type `platform::platform::x11::util::input::PointerState` uninitialized, which is invalid'

To reproduce this panic, just launch the example and move the mouse over the window. It happens with rust 1.48 and above.

See rust-windowing/winit#1778

Just bumping piston_window to 0.120.0 fixes the issue